Surfactants based on hydrocarbyl glycosides are gaining increasing attention due to their advantages over other surfactants with regard to notable dermatological properties and good compatibility with standard products, as well as their favorable environmental profile. In addition, their low toxicity, good biocompatibility and fast biodegradation make this class of molecules very attractive, not only for personal care products but also for a large range of technical applications.
Compositionally, hydrocarbyl glycosides are fatty alcohol-glycose adducts which are covalently bound by an acetal linkage at the anomeric carbon of the glycose moiety. These can be used as nonionic surfactants that provide detergency, foaming, emulsifying, and wetting properties that are comparable to those of other nonionic surfactants.
In general, two different processes are practiced in the commercial production of alkyl polyglucosides, direct glycosidation and transglycosidation. Both manufacturing processes operate via Brönsted acid-catalyzed condensation at the anomeric (aldehydic) carbon of a monosaccharide, commonly referred to as Fischer glycosidation.
Direct glycosidation can be characterized by the coupling of a high molecular weight alcohol with glycose, without the formation of an intermediate low molecular weight alcohol-glycose adduct.
Transglycosidation, conversely, proceeds through a comparatively lower molecular weight, transient alkyl glycoside en route to the desired alkyl glycoside product.
Improvements in metathesis catalysts (see J. C. Mol, Green Chem. 4 (2002) 5) provide an opportunity to generate reduced chain length, monounsaturated feedstocks, which are valuable for making detergents and surfactants, from C16 to C22-rich natural oils such as soybean oil or palm oil. Soybean oil and palm oil can be more economical than, for example, coconut oil, which is a traditional starting material for making detergents. As this article explains, metathesis relies on conversion of olefins into new products by rupture and reformation of carbon-carbon double bonds mediated by transition metal carbene complexes. Self-metathesis of an unsaturated fatty ester can provide an equilibrium mixture of starting material, an internally unsaturated hydrocarbon, and an unsaturated diester. For instance, methyl oleate (methyl cis-9-octadecenoate) is partially converted to 9-octadecene and dimethyl 9-octadecene-1,18-dioate, with both products consisting predominantly of the trans-isomer. Metathesis effectively isomerizes the cis-double bond of methyl oleate to give an equilibrium mixture of cis- and trans-isomers in both the “unconverted” starting material and the metathesis products, with the trans-isomers predominating.
Cross-metathesis of unsaturated fatty esters with olefins generates olefins and unsaturated esters that can have reduced chain length and that may be difficult to make otherwise. For instance, cross-metathesis of methyl oleate and 3-hexene provides 3-dodecene and methyl 9-dodecenoate (see also U.S. Pat. No. 4,545,941). Terminal olefins are particularly desirable synthetic targets. Elevance Renewable Sciences, Inc. has described an improved way to prepare them by cross-metathesis of an internal olefin and an α-olefin in the presence of a ruthenium alkylidene catalyst (see U.S. Pat. Appl. Publ. No. 2010/0145086). A variety of cross-metathesis reactions involving an α-olefin and an unsaturated fatty ester (as the internal olefin source) are described. Thus, for example, reaction of soybean oil with propylene followed by hydrolysis gives, among other things, 1-decene, 2-undecene, 9-decenoic acid, and 9-undecenoic acid.